U.S. patent application number 11/578343 was filed with the patent office on 2007-11-29 for nuclear magnetic resonance apparatus.
This patent application is currently assigned to ESAOTE, S.P.A.. Invention is credited to Eugenio Biglieri, Stefano Pittaluga, Luigi Satragno, Alessandro Trequattrini.
Application Number | 20070273378 11/578343 |
Document ID | / |
Family ID | 34956265 |
Filed Date | 2007-11-29 |
United States Patent
Application |
20070273378 |
Kind Code |
A1 |
Trequattrini; Alessandro ;
et al. |
November 29, 2007 |
Nuclear Magnetic Resonance Apparatus
Abstract
A magnet for a Magnetic Resonance Imaging apparatus, said magnet
having at last two, preferably three open sides, and delimiting the
patient receiving area, said magnet being composed of three yoke
elements, i.e. two parallel magnetically permeable elements and one
transverse yoke element for connection of said two parallel
elements, whereby said magnet has a C shape and wherein, for both
of said parallel elements, the terminal portion associated to said
transverse connecting element has a transverse wall for connection
of said two parallel elements with the transverse element, which
has an end step for engagement of a corresponding end of said
transverse element.
Inventors: |
Trequattrini; Alessandro;
(Genova, IT) ; Satragno; Luigi; (Genova, IT)
; Biglieri; Eugenio; (Masio, IT) ; Pittaluga;
Stefano; (Genova, IT) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
ESAOTE, S.P.A.
Milano
IT
I-20123
|
Family ID: |
34956265 |
Appl. No.: |
11/578343 |
Filed: |
March 25, 2005 |
PCT Filed: |
March 25, 2005 |
PCT NO: |
PCT/EP05/51399 |
371 Date: |
October 12, 2006 |
Current U.S.
Class: |
324/318 ;
324/307; 324/316 |
Current CPC
Class: |
G01R 33/383
20130101 |
Class at
Publication: |
324/318 ;
324/307; 324/316 |
International
Class: |
G01R 33/20 20060101
G01R033/20; G01R 33/381 20060101 G01R033/381 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2004 |
IT |
SV2004A000016 |
Claims
1. A magnet for a Magnetic Resonance Imaging apparatus, said magnet
having at least two open sides, and delimiting a patient receiving
area, said magnet comprising: two parallel magnetically permeable
yoke elements; and a transverse yoke element for connection of said
two parallel elements, said magnet thus having a C shape, wherein
in both parallel elements, an end portion associated to said
transverse connection element has a transverse wall for connecting
the two parallel elements with the transverse element, which has an
end step for engagement of a corresponding end of said transverse
element.
2. A magnet as claimed in claim 1, further comprising fastener
means which are oriented perpendicular to a longitudinal extension
of said transverse wall, and which engage the two parallel elements
with the transverse element within a raised thickness of said end
step.
3. A magnet as claimed in claim 1, further comprising fastener
means which are oriented parallel to a longitudinal extension of
said transverse wall, and which engage the two parallel elements
with said transverse element at said end step.
4. A magnet as claimed in claim 1, further comprising reinforcement
members positioned in inwardly facing corners between said parallel
elements and said transverse element, and fastener means between
said reinforcement members and said parallel elements and/or said
transverse element for fastening and holding in position said
reinforcement members.
5. A magnet as claimed in claim 4, wherein said reinforcement
members are formed by at least one single triangular member.
6. A magnet as claimed in claim 5, wherein said triangular member
is provided for each parallel element, and extends essentially
along the whole parallel element and transverse element coupling
length.
7. A magnet as claimed in claim 4, wherein said reinforcement
member is formed by one or more triangular reinforcing ribs.
8. A magnet as claimed in claim 7, wherein two triangular
reinforcing ribs are provided for each parallel element/transverse
element pair.
9. A magnet as claimed in claim 4, wherein said fastener means are
provided in the form of screw fasteners.
10. A pole piece for a Magnetic Resonance Imaging apparatus,
comprising at least one magnetized layer; and at least one
magnetically permeable layer or plate, wherein said magnetically
permeable plate has a substantially circular shape and said
magnetized layer is formed by a plurality of adjacent and/or
superimposed magnetized blocks, which form a polygon essentially
approximating the circular shape of said magnetically permeable
plate.
11. A pole piece as claimed in claim 10, further comprising
fastener posts located in a margin between said polygon and said
circumference approximated by the polygon, for securing the pole
piece to an inner face of a corresponding parallel yoke
element.
12. A pole piece as claimed in one claim 11, wherein each of said
blocks has a cubical shape or a rectangular section such that the
blocks form a polygonal layer with stepped peripheral edges, the
fastener posts being located in recesses of said stepped peripheral
edges.
13. A pole piece as claimed in claim 11, wherein said posts have a
tubular shape.
14. A pole piece as claimed in claim 13, wherein said magnetically
permeable plate has centering recesses for positioning said posts,
wherein corresponding post ends may be introduced, said centering
recesses being complementary in size to an outside diameter of the
posts and/or to an outside diameter of the associated post end.
15. A pole piece as claimed in claim 14, further comprising a pole
piece support structure, centering recesses being provided on the
support structure surface facing toward said pole piece, for the
associated fastener posts of the pole piece.
16. A pole piece as claimed in claim 11, wherein said fastener
posts of the pole piece have a tubular shape and contain a hole for
the passage of an associated fastener means, such as a bolt and/or
a screw.
17. A pole piece as claimed in claim 15, wherein said fastener
posts of the pole piece have a reduced or increased diameter at
their free ends, corresponding to a diameter of the associated
centering recesses and/or to the corresponding diameter of the
positioning recesses, to act as spacers with a predetermined stable
size, the end surfaces of said posts being secured against the
corresponding surfaces of the bottom sides of said recesses.
18. A pole piece as claimed in claim 10, further comprising a sheet
pack, which is formed by at least one metal sheet with cuts
thereon, which is positioned on the surface of the magnetically
permeable plate opposite the surface of the plate which is turned
toward the magnetized layer.
19. A pole piece as claimed in claim 18, wherein said magnetically
permeable plate has a cup shape, having at least one peripheral
step, which forms a raised edge projecting out of the plate surface
opposite the magnetized layer, so that the sheet pack lies on the
bottom of the cup-shaped plate.
20. A pole piece as claimed in claim 19, wherein said sheet pack
lies under a shimming plate which peripherally lies on a second
step, at a distance from the bottom of the plate equaling a raised
thickness of the sheet pack, and lying sideways on a raised portion
of the second step.
21. A pole piece as claimed in claim 18, wherein each metal sheet
of said sheet pack has a plurality of cuts, the cuts of one sheet
being located in different positions with respect to the cuts of a
preceding or subsequent sheet in the sheet pack.
22. A pole piece as claimed in claim 21, wherein said cuts on each
of said metal sheets are essentially straight and non coincident
between even sheets and odd sheets, with reference to a sheet
positioning order in the sheet pack.
23. A pole piece as claimed in claim 10, wherein pole piece heaters
and temperature sensors are housed within said pole pieces.
24. A pole piece as claimed in claim 23, wherein cables for the
pole piece heaters and the temperature sensors are housed within
the pole piece, suitable apertures being formed in the peripheral
edges of said pole piece for the passage of said cables for the
heaters and/or said temperature sensors.
25. A pole piece as claimed in claim 24, wherein said apertures are
two passages formed on diametrically opposite locations of said
plate.
26. A pole piece as claimed in claim 18, wherein at least a few
electric heaters are placed in contact with said plate and/or with
said magnetized material and/or with said sheet pack.
27. A pole piece as claimed claim 26, wherein at least some of said
heaters are located on externally accessible pole piece
surfaces.
28. A pole piece as claimed in claim 26, wherein at least some of
said heaters are located within the magnetized layer.
29. A pole piece as claimed in claim 26, wherein at least some of
said heaters are located between adjacent magnetized blocks.
30. A pole piece as claimed in claim 27, wherein said heaters
located on the externally accessible pole piece surfaces are
peripheral band-like heaters positioned on a lateral peripheral
surface or edge of the pole piece.
31. A pole piece as claimed in claim 30, wherein said peripheral
band-like heaters are continuously positioned all along the lateral
peripheral surface of the pole piece.
32. A pole piece as claimed in claim 30, wherein said peripheral
band-like heaters are positioned at least partly along the lateral
peripheral surface of the pole piece.
33. A pole piece as claimed in claim 26, wherein said heaters are
flat heaters located on at least one face of the pole piece.
34. A pole piece as claimed in claim 33, wherein said flat heaters
have a D shape resembling sectors of a circle.
35. A pole piece as claimed in claim 34, wherein said flat D-shaped
heaters are spaced over the face of the pole piece, to form an area
therebetween which is not directly heated.
36. A pole piece as claimed in claim 18, wherein at least two
temperature sensors are provided on the same pole piece.
37. A pole piece as claimed in claim 36, wherein said pole piece
temperature sensors are located level with said plate and/or with
said magnetized material and/or with said sheet pack.
38. A pole piece as claimed in claim 36, wherein at least some of
said temperature sensors are located within the magnetized layer,
preferably between adjacent magnetized blocks.
39. A Magnetic Resonance imaging apparatus, comprising: a magnet
having at least two pole pieces, between which pole pieces a cavity
is formed for containing at least one body under examination,
wherein said cavity is open on at least one side; and at least one
magnet support structure, wherein said magnet is capable of
rotating relative to said support structure.
40. A Magnetic Resonance Imaging apparatus as claimed in claim 39,
wherein the magnet rotates about an axis perpendicular to at least
one axis of said cavity opening.
41. A Magnetic Resonance Imaging apparatus as claimed in claim 40,
wherein said axis of rotation essentially passes through the center
of an imaging volume of the apparatus, and wherein said imaging
volume includes a portion of a magnetic field within said cavity
which has the best characteristics for imaging the body under
examination.
42. A Magnetic Resonance Imaging apparatus as claimed in claim 39,
wherein said magnet has a flange in its transverse wall opposite
the cavity, for connection between a shaft and said transverse wall
of said magnet, said shaft being rotatably mounted to a support
case by rotatable and translatable supporting means.
43. A Magnetic Resonance Imaging apparatus as claimed in claim 42,
wherein the coupling flange and the support case have mutually
cooperating lock/release means for locking/releasing the rotation,
enabling a change from a magnet rotation condition to a magnet stop
condition.
44. A Magnetic Resonance Imaging apparatus as claimed in claim 43,
wherein said lock/release means are provided in the form of two
concurrent ring gears, wherein one of the ring gears is non
rotatably connected to the case and the other is connected to the
coupling flange in such a manner as to rotate therewith, said ring
gears being maintained in a stable mutual engagement condition by a
force exerted by an elastic member, the apparatus further
comprising means for moving the two ring gears to a disengagement
condition against the force of said elastic member.
45. A Magnetic Resonance Imaging apparatus as claimed in claim 44,
wherein said elastic member is a Belleville spring.
46. A Magnetic Resonance Imaging apparatus as claimed in claim 44,
wherein said ring gears have front teeth.
47. A Magnetic Resonance Imaging apparatus as claimed in claim 44,
wherein the ring gear associated to the flange is integral with the
shaft.
48. A Magnetic Resonance Imaging apparatus as claimed in claim 42,
wherein the shaft has a shaft handling grip on its outer
surface.
49. A Magnetic Resonance Imaging apparatus as claimed in claim 48,
wherein said handling grip is a radial lever interposed between
said rotatable support means, or bearings.
50. A Magnetic Resonance Imaging apparatus as claimed in claim 42,
further comprising at least one radial lever, which is rotatably
integral with the shaft, and is articulated to the end of a linear
actuator.
51. A Magnetic Resonance Imaging apparatus as claimed in claim 45,
wherein the ring gear associated to the shaft by said flange and
the Belleville spring are configured to be respectively displaced
and stressed in the axial direction of the shaft.
52. A Magnetic Resonance Imaging apparatus as claimed in claim 44,
wherein the means for moving one or both ring gears are hydraulic
or pneumatic lock/release means.
53. A Magnetic Resonance Imaging apparatus as claimed in claim 44,
wherein an end of the shaft opposite the magnet is a hydraulic or
pneumatic lock/release cylinder actuator, having a rod that
projects out of said end of the shaft, wherein the rod is mounted
so as to be able to rotate relative to the shaft and is alternately
subjected to the force of the elastic means, which opposes shaft
translation toward disengagement from the ring gear.
54. A Magnetic Resonance Imaging apparatus as claimed in claim 44,
wherein an end of the shaft opposite the magnet has a coaxial dead
recess, which acts as a chamber for a hydraulic or pneumatic
lock/release cylinder actuator, a piston being received therein,
which is mounted at an inner end of a rod, the cylindrical recess
being tightly closed by an end portion at a head of the shaft, and
a spring being provided between the piston and said end portion,
wherein the piston tightly projects out of said end portion.
55. A Magnetic Resonance Imaging apparatus as claimed in claim 50,
wherein the radial lever is coupled with the rod of the actuator by
sliding means, which slide in the axial shaft translation
direction.
56. A Magnetic Resonance Imaging apparatus as claimed in claim 55,
wherein the lever has a perforated and thinned terminal, which is
mounted on a bar that is held at its ends by a terminal fork for
connection with the rod.
57. A Magnetic Resonance Imaging apparatus as claimed in claim 56,
wherein said fork has a width such that the bar held thereby is at
least as long as or slightly longer than the translation stroke of
the shaft.
58. A Magnetic Resonance Imaging apparatus as claimed in claim 50,
wherein the linear actuator has a slide guide at its end associated
to the support case, enabling translational motion of the
actuator.
59. A Magnetic Resonance Imaging apparatus as claimed in claim 58,
wherein a rod of the actuator has one degree of freedom of
rotation, with respect to the radial lever.
60. A Magnetic Resonance Imaging apparatus as claimed in claim 59,
wherein as the shaft is translated, the shaft also drives into
translation the linear actuator, which follows the shaft by
translating on the slide guide that connects it to the support
case.
61. A Magnetic Resonance Imaging apparatus as claimed in claim 50,
wherein the linear actuator is associated to the support case or to
the radial lever with the interposition of a joint which allows the
linear actuator to tilt to accommodate the shaft translation.
62. A Magnetic Resonance Imaging apparatus as claimed in claim 50,
wherein the radial lever has a joint which allows the radial lever
to tilt with respect to the shaft, to accommodate the translational
motion of the shaft with respect to the actuator.
63. A patient support device or table for a Magnetic Resonance
Imaging apparatus, comprising guides along which the patient
support device may slide with respect to the structure of the
associated Magnetic Resonance Imaging apparatus, which are located
at the sides of said patient support device.
64. A patient support device or table for a Magnetic Resonance
Imaging apparatus as claimed in claim 63, wherein said guides
comprise a vertical support surface which extends from the sides of
the patient table toward the pole piece, said support surface being
associated to a guide and slide combination.
65. A patient support device or table for a Magnetic Resonance
Imaging apparatus as claimed in claim 64, wherein said guide and
slide combination is a slide having a continuous or discontinuous
semi-cylindrical head and cooperating with a continuous or
discontinuous concave guide which has a corresponding open
profile.
66. A Magnetic Resonance Imaging apparatus as claimed in claim 39,
wherein the magnet includes two parallel magnetically permeable
yoke elements, and a transverse yoke element for connection of said
two parallel elements, said magnet thus having a C shape, wherein
in both parallel elements, an end portion associated to said
transverse connection element has a transverse wall for connecting
the two parallel elements with the transverse element, which has an
end step for engagement of a corresponding end of said transverse
element, and wherein each pole piece includes at least one
magnetized layer, and at least one magnetically permeable layer or
plate, wherein said magnetically permeable plate has a
substantially circular shape and said magnetized layer is formed by
a plurality of adjacent and/or superimposed magnetized blocks,
which form a polygon essentially approximating the circular shape
of said magnetically permeable plate, the apparatus further
comprising a patient support device including guides along which
the patient support device may slide with respect to the structure
of the associated Magnetic Resonance Imaging apparatus, which are
located at the sides of said patient support device.
67. A magnet according to claim 66, wherein the magnet includes
permanent magnets or resistive magnetic field generating means or
superconducting generating means.
68. A Magnetic resonance imaging apparatus according to claim 39,
wherein the magnet comprises permanent magnets or resistive
magnetic field generating means or superconducting generating
means.
69. A Magnetic resonance imaging apparatus comprising the pole
piece according to claim 10 and permanent magnets or resistive
magnetic field generating means or super-conducting generating
means.
Description
[0001] This invention relates to a Magnetic Resonance Imaging
apparatus, comprising at least: a magnet having at least two pole
pieces, between which pole pieces a cavity is formed for containing
at least one body under examination, which cavity is open on at
least one side, and at least one magnet support structure.
[0002] Magnetic Resonance Imaging apparatuses are known to be quite
heavy, whereby rotary handling of the magnet is often difficult.
The magnet is often required to be rotated for examinations which
could not be easily performed on patients with limited mobility or
on certain body parts. Thus, the magnet must be easily rotated to
positions other than the normal horizontal patient position, either
through a few degrees or through rotation angles greater than
90.degree..
[0003] Therefore, there is a need to provide a Magnetic Resonance
Imaging apparatus which features a very simple construction and a
high strength and safety. In addition to being bulky, Magnetic
Resonance Imaging apparatuses are also known to be quite heavy,
which poses serious magnet handling problems, as well as related
safety drawbacks. An accidental and uncontrolled rotation of the
magnet might have dangerous effects both for the patient under
examination and for any operator moving in the operating range of
the apparatus.
[0004] The object of this invention is to provide a magnet for a
Magnetic Resonance Imaging apparatus, which might simply and
inexpensively obviate the drawbacks of prior art Magnetic Resonance
Imaging apparatuses, thereby providing a simple, inexpensive and
reliable, as well as highly fail-safe apparatus.
[0005] The invention fulfils the above objects by providing an
apparatus for Nuclear Magnetic Resonance imaging, comprising at
least: a magnet having at least two pole pieces, between which pole
pieces a cavity is formed for containing at least one body under
examination, which cavity is open on at least one side, and at
least one magnet support structure, and wherein said magnet may be
rotated relative to this support structure.
[0006] According to an advantageous embodiment of the inventive
apparatus, the magnet rotates about an axis perpendicular to at
least one axis of such cavity opening, particularly this axis of
rotation essentially passes through the center of the imaging
volume of the apparatus, where imaging volume, as used herein,
refers to the portion of the magnetic field within the cavity which
has optimal characteristics for imaging the body under
examination.
[0007] In the Magnetic Resonance Imaging apparatus according to a
preferred embodiment of the invention, the magnet has a flange in
its transverse wall opposite the cavity, for connection with a
shaft, the shaft being mounted to a support case in such a manner
as to be able to be rotated and translated horizontally, by
rotatably and translatably supporting means.
[0008] Removable lock/release means are provided between the shaft,
which is rotatably and translatably connected to the magnet, and
the support case, which allow to change from a magnet rotation
condition to a magnet stop condition. According to a non limiting
embodiment, the lock/release means are formed, for instance, by two
opposed ring gears, whereof one is non rotatably connected to the
support case and the other is secured to the shaft in such a manner
as to rotate therewith, the two ring gears being mutually
engageable and disengageable by axial translational motion of the
shaft relative to the case.
[0009] The ring gears preferably have front teeth and are
maintained in a stable mutual engagement condition thanks to the
force exerted by an elastic member, whereas means are provided for
moving the two ring gears to a disengagement condition against the
action of the elastic member. The elastic member is preferably
provided in the form of a Belleville spring, whereas the
displacement in the disengagement direction is exerted by an
actuator of any type whatever, particularly a hydraulic or
pneumatic actuator. Advantageously, the hydraulic actuator is at
least partly integrated in the shaft.
[0010] Motion can be transmitted to the shaft by providing a shaft
handling grip point on the outer surface thereof, which grip is
formed by a radial lever, which is itself translatable with the
shaft, whereby any magnet translational motion causes a
corresponding translational motion of the radial lever. The shaft
is supported in the case by bearings, which are preferably placed
at the ends of the shaft, and are, for instance, roller
bearings.
[0011] The radial lever is connected to the end of the rod of a
hydraulic or pneumatic rotary cylinder actuator, i.e. a linear
actuator, which transmits motion to the shaft.
[0012] In one embodiment, the radial lever is coupled to the rod of
the actuator by sliding means, which slide in the axial shaft
translation direction, e.g. the lever has a thinned perforated
terminal, which is mounted on a bar that is held at its ends by a
terminal fork for connection with the rod. The fork has such a
width that the bar retained thereby is at least as long as or
slightly longer than the translation stroke of the shaft.
Therefore, a translational motion of the shaft and the associated
radial lever does not involve an identical translational motion of
the actuator, which stands still with respect to the case, as the
radial lever slides on the bar retained by the fork of the linear
actuator.
[0013] In an alternative embodiment, the linear actuator has a
slide guide at its end associated to the support case, for allowing
translational motion of the actuator, and the rod of the actuator
has one degree of freedom, particularly of rotation, with respect
to the radial lever. Hence, as the shaft is translated, it also
drives into translation the linear actuator, which follows the
shaft by translating on the slide guide that connects it to the
support case.
[0014] In a further alternative embodiment, the linear actuator is
connected to the support case or to the radial lever with the
interposition of a joint which allows it to tilt to accommodate the
shaft translation.
[0015] This is possible because such translational motion extends
for so little, i.e. in a range of a few millimeters, that the
substantial verticality of the linear actuator is not dramatically
affected. As an alternative, the radial lever may be designed to
have a joint which allows it to tilt relative to the shaft, to
accommodate the translational motion of the shaft relative to the
actuator.
[0016] The ring gears and the Belleville spring may be displaced
and stressed in the axial direction of the shaft by hydraulic or
pneumatic lock/release displacement means. In a preferred
embodiment, the end of the shaft opposite the magnet is provided in
the form of a hydraulic or pneumatic lock/release cylinder
actuator, whose rod projects out of this end of the shaft, whereas
the rod is mounted in such a manner as to be able to rotate
relative to the shaft and is alternately subjected to the action of
elastic means, which oppose the shaft translation toward
disengagement of the ring gear teeth.
[0017] The angular position of the shaft and of the associated ring
gear relative to the case, and to the associated ring gear may be
controlled by sensors and/or photocells and/or encoders which
ensure that shaft rotation only stops when the facing teeth of the
ring gears are in a proper mutually meshed position. As an
alternative thereto or in combination therewith the magnet extreme
positions may be also monitored, so that shaft rotation
automatically stops when the shaft reaches either extreme position,
i.e. when the magnet is in either C- or U-shaped position.
[0018] This preferred arrangement provides a Magnetic Resonance
Imaging apparatus which has a simple construction and is highly
fail-safe, as the magnet may be only released when the two ring
gears are moved to a disengagement position and allow free rotation
of the magnet. Therefore, the actuation of the linear rotary
actuator controls the rotation of the shaft and of the magnet into
the desired position. As soon as the desired rotation is completed,
the ring gears move back into the contact position, with aligned
teeth for proper meshing, and are locked together thanks to the
action of the Belleville spring which compresses them against each
other, and of the sensors and/or photocells and/or encoders which
are possibly provided to monitor the relative positions of the
teeth. As pressure is released in the hydraulic lock/release
cylinder, in case of a sudden failure of the apparatus or a sudden
power failure, the action of the Belleville spring automatically
moves the ring gears into a mutual engagement position, thereby
causing the magnet to be immediately locked in position, and
preventing any uncontrolled or undesired rotation thereof.
[0019] A further advantage as compared with prior art apparatuses
is that any magnet lock/release and rotation action is performed by
linear actuators, which have a simpler construction and ensure a
higher reliability at lower costs as compared with normal prior art
actuators.
[0020] Furthermore, the present invention addresses, separately
from or in combination with the above, a magnet for a Magnetic
Resonance Imaging apparatus, said magnet having at least two,
preferably three open sides, and delimiting the patient receiving
area; said magnet being further composed of three yoke elements,
i.e. two parallel magnetically permeable yoke elements, and a
transverse yoke element for connection of said two parallel
elements, whereby said magnet has a C shape.
[0021] Such magnets are well-known and widely used. While these
magnets satisfactorily serve their function, they still suffer from
certain drawbacks. It is well known to the skilled person that
magnet structures of Magnetic Resonance Imaging apparatuses are
quite heavy and pose problems in rotary handling of magnets. An
important characteristic of magnet structures is that they generate
a highly homogeneous magnetic field between the pole pieces in at
Least a portion of the overall volume of the cavity delimited by
the magnetic structure (i.e. imaging volume). Such high homogeneity
is imperatively required for the images of the body being examined
to be as reliable as possible. Therefore, the relative position of
the pole pieces is a very critical factor, as even the slightest
offset from the design relative position of the pole pieces would
alter the magnetic field lines, thereby affecting the magnetic
field or introducing inhomogeneities therein. In all Magnetic
Resonance Imaging apparatuses in which the magnet structure always
has the same orientation with respect to gravity, any offset from
the ideal position of the pole pieces, caused by gravity or by
mutual magnetic attraction between the opposed pole pieces is
accounted and compensated for during fabrication.
[0022] Such compensation may be performed once and for all, as the
magnetic and gravity forces which act in the magnet structure are
constant.
[0023] However, particularly for essentially C- or U-shaped magnet
structures, problems arise when rotating the magnet for the
examination of a body in certain positions or of certain body
parts. As they rotate, the magnet structure, and particularly the
pole piece supporting elements thereof, e.g. the two opposed yoke
plates of a C-shaped magnet, change their orientation relative to
gravity, therefore the stresses acting thereon also change.
[0024] In a C-shaped magnet, which moves from a position in which
the opposed pole piece-supporting yoke plates are oriented
horizontally into a position in which said yoke plates are oriented
vertically, the bending stress exerted by gravity passes from being
perpendicular to the pole pieces to being parallel to the surfaces
of the pole pieces. Here, the compensations provided during
fabrication for the horizontal position will not apply to the
rotated magnet, with the yoke plates in the vertical position.
[0025] If the magnet is a stationary magnet, mechanical prestress
loading may be provided to compensate for bending stresses, but
this arrangement is not applicable to a rotating magnet.
[0026] This condition becomes intolerable when the magnet, for a
number of different reasons, is to be rotated through angles
greater than 90.degree..
[0027] The object of this invention is to provide a magnet for a
Magnetic Resonance Imaging apparatus, which might simply and
inexpensively obviate the drawbacks of prior art magnets, while
maintaining both a small size and a light weight of the magnet
structure.
[0028] The invention fulfils the above objects by providing a
magnet for a Magnetic Resonance Imaging apparatus, said magnet
having at last two, preferably three open sides, and delimiting the
patient receiving area; said magnet being composed of three yoke
elements, i.e. two parallel magnetically permeable elements and one
transverse yoke element for connection of said two parallel
elements; whereby said magnet has a C shape and wherein, for both
of said parallel elements, the terminal portion associated to said
transverse connecting element has a transverse wall for connection
of said two parallel elements with the transverse element, which
has an end step for engagement of a corresponding end of said
transverse element.
[0029] Particularly, in an advantageous embodiment of the inventive
magnet, fastener means are provided which are oriented
perpendicular to the longitudinal extension of said transverse
wall, which fastener means engage the two parallel elements with
the transverse element within the raised thickness of the end step.
Fastener means are further provided parallel to the longitudinal
extension of said transverse wall, which engage the two parallel
elements with the transverse element at the end step.
[0030] Therefore, the magnet of this invention provides a high
strength as well as a highly simple construction.
[0031] In order to further improve the strength of the structure,
especially when the magnet is intended to be rotated, in a
preferred embodiment further reinforcement members are provided in
the inwardly facing corners between the parallel elements and the
transverse element, fastener means being provided between the
reinforcement members and the parallel elements and/or the
transverse element, for fastening and holding in position said
reinforcement members. The reinforcement members may advantageously
be at least one single triangular member for each parallel element,
extending essentially along the whole parallel element and
transverse element coupling length. Nevertheless, according to an
alternative embodiment, the reinforcement member may be formed by
one or more reinforcing ribs, preferably having a triangular shape,
to provide better magnet weight reduction results. Particularly, an
optimal arrangement was found to comprise two triangular
reinforcing ribs for each parallel element/transverse element
pair.
[0032] The fastener means of the parallel elements, of the
transverse elements and of the reinforcing ribs may be screw
fasteners, which provide accurate, and optimally firm
positioning.
[0033] The magnet of this invention advantageously and essentially
allows to avoid any relative displacement between the parts which
compose the magnet so that, even upon rotation, the imaging volume
is essentially unaltered and optimal for a proper examination of
the body in any position thereof. In fact, the inventive magnet has
a highly stable structure, and the deformation of the parts which
compose the C-shaped magnet is insignificant.
[0034] As an alternative to or in combination with the above, this
invention also addresses a magnetic pole piece for a Magnetic
Resonance Imaging apparatus, comprising at least one magnetized
layer, and at least one magnetically permeable layer or plate.
[0035] Image quality is known to be related to the homogeneity and
constancy of the magnetic field with time. This is ensured by
proper relative positioning of the pole pieces and by the
construction and temperature of the latter.
[0036] In fact, the pole pieces of a Magnetic Resonance imaging
apparatus are composed of several parts, which shall be held in
proper mutual positions, even when the magnet, and the pole pieces,
are intended to rotate. During rotation, the direction of the
gravity force acting on the pole pieces of the magnet changes with
respect to the axis of the pole pieces and, as each pole piece is
formed by several parts, its components must remain in a constant
relative position, regardless of their position relative to the
gravity force.
[0037] Furthermore, the temperature of the pole pieces must be kept
constant, to avoid any magnetic field drift, and any shift of the
precession frequency of nuclear spins.
[0038] A further object of this invention is to provide pole pieces
for a Magnetic Resonance imaging apparatus, which have a simple and
sturdy construction, capable of withstanding stresses in several
different directions without being deformed and with no relative
displacement of the pole piece components.
[0039] Yet another object of this invention is to provide pole
pieces having optimal arrangements for pole piece heating and
temperature monitoring.
[0040] The invention fulfils the above objects by providing a pole
piece for a Magnetic Resonance Imaging apparatus, which comprises
at least one magnetized layer, at least one magnetically permeable
layer or plate, wherein said magnetically permeable layer or plate
has a substantially circular shape and said magnetized layer is
formed by a plurality of adjacent and/or superimposed magnetized
blocks, which form a polygon essentially approximating the circular
shape of said magnetically permeable plate.
[0041] In accordance with a preferred embodiment, the magnetic pole
piece for Magnetic Resonance Imaging apparatuses further provides
fastener posts for securing the pole piece to the inner face of the
corresponding parallel yoke element, which are placed in the margin
between the polygon and the circumference approximated by the
polygon. Each block has a polygonal, particularly cubical shape or
a rectangular section, and together form a polygonal layer with
stepped peripheral edges and the fastener posts are placed in the
recesses of these stepped peripheral edges.
[0042] The posts preferably have a tubular shape and contain a hole
for the passage of an associated fastener means, such as a bolt
and/or a screw.
[0043] The magnetically permeable plate has centering recesses for
positioning the posts, wherein corresponding post ends may be
introduced. The centering recesses are complementary in size to the
outside diameter of the posts and/or to the outside diameter of the
associated end of the post. The pole piece support structure also
has centering recesses for the associated fastener posts of the
pole piece. The fastener posts of the pole piece have a reduced or
increased diameter at their free ends, corresponding to the
diameter of the associated centering recesses, to act as spacers
with a predetermined stable size, the end surfaces of the posts
being secured against the corresponding surfaces of the bottom
sides of the recesses. Advantageously this arrangement provides a
highly sturdy and compact pole piece, which may be easily rotated
with no relative displacement of its parts, and no imaging volume
alteration.
[0044] In this embodiment, a sheet pack is further provided, which
is formed by at least one metal sheet with cuts thereon, which is
positioned on the surface of the magnetically permeable plate
opposite the surface of the plate which is turned toward the
magnetized layer. The sheet pack is advantageously placed at a
distance from the facing surface of the plate, thereby forming a
hollow space therewith. The magnetically permeable plate has a cup
shape, having at least one, preferably two peripheral steps, which
form a raised edge projecting out of the plate surface facing
toward the magnetized layer, so that the sheet pack lies on the
bottom of the cup-shaped plate. Suitable housings are
advantageously formed in the pole piece for the pole piece heaters
and temperature sensors.
[0045] Furthermore, the pole piece also houses the cables for the
pole piece heaters and temperature sensors. Suitable apertures are
formed in the peripheral edges of the pole piece for the passage of
the cables for the heaters and/or the temperature sensors, and
preferably these apertures are two apertures formed on
diametrically opposite locations of the plate.
[0046] For heating purposes, at least one or more electric heaters
are placed in contact with the plate and/or with the magnetized
material and/or with the sheet pack.
[0047] In a preferred embodiment, the heaters are placed within the
magnetized layer, particularly at least some of the heaters are
positioned between adjacent magnetized blocks, to maintain their
temperature constant and to provide a more effective heating.
[0048] In a further embodiment, the heaters are advantageously
placed in areas of the pole piece that may be accessed from outside
the pole piece, and be easily serviced and replaced during
maintenance, by only removing/reaffixing the covering.
Particularly, the heater may be formed by a peripheral heating band
extending on the side surface of the pole piece edge. This
peripheral heating band may be continuous or discontinuous,
depending on construction requirements.
[0049] Alternatively thereto or in combination therewith, two flat
D-shaped heaters may be provided on the inner flat surface of the
pole piece, which provide a uniform heating of the pole piece.
[0050] The temperature sensors for monitoring the pole piece
temperature are situated level with the plate and/or the magnetized
material and/or the sheet pack, to effectively monitor the
temperature of the various parts of the pole piece and indicate any
abnormal condition.
[0051] Particularly, the temperature sensors are positioned level
with the magnetized layer and preferably between adjacent
magnetized blocks, to be able to effectively monitor the
temperature within the layer of magnetized blocks.
[0052] In accordance with a preferred embodiment, two of these
temperature sensors may be provided, placed inside the pole piece,
particularly between the blocks and the sheet pack of the pole
piece, in the area that is not directly heated by these two flat
D-shaped heaters. Therefore, temperature is sensed in an area that
is not directly heated by heaters, whereby a more reliable
detection is obtained, the temperature of the whole pole piece
being more indicative than the temperature of a portion of it.
[0053] Furthermore, the presence of two temperature sensors
advantageously avoids the need of dismantling the whole pole piece
for sensor replacement, in case of failure or malfunction of one of
the two pole pieces. In a prior art apparatus with a single
temperature sensor, a failure of the latter requires the whole
apparatus to be shut down, the pole piece to be completely
dismantled and the faulty temperature sensor to be replaced,
whereupon the pole piece has to be reassembled, and the shimming
plate has to be recalibrated by repositioning the permanent magnets
thereon, to obtain the desired magnetic field homogeneity.
[0054] The provision of two temperature sensors, which are
typically low-cost parts, allows the apparatus to continue
operating normally, in case of failure of a sensor, and to require
no expensive and complex operation of pole piece dismantling and
reassembly, the whole with little expenses.
[0055] This invention also relates, alternatively to or in
combination with the above, to a patient support table associated
to the lower pole piece or to the lower arm of the C-shaped magnet
which forms a Magnetic Resonance Imaging apparatus, which table is
slideably associated to the structure of the apparatus by slide
guides placed at the sides of the pole piece. These guides allow a
fast and safe handling of the patient table or support device,
which may slide relative to the surface of the pole piece with
which it is associated, and is locked in position when needed. This
arrangement allows proper positioning of the patient or the
relevant body parts under examination. The provision of lateral
slide guides advantageously provides a very compact apparatus, the
height of the pole piece and the patient table being unaffected by
the thickness of the slide guides, unlike prior art apparatuses, in
which slide guides are typically positioned on the pole piece, i.e.
between the patient table and the pole piece.
[0056] Although many of the features disclosed above are referred
to a permanent magnet, some features such as the features of the
construction which enables the magnetic structure to rotate can
also be provided in combination with a resistive or superconducting
magnetic structure. Also the construction of the joke can be
provided in combination with means for generating the magnetic
field which are resistive or superconducting.
[0057] Further characteristics and improvements will form the
subject of the annexed claims.
[0058] The characteristics of the invention and the advantages
derived therefrom will be more apparent from the following detailed
description of the accompanying drawings, in which:
[0059] FIG. 1 is a side view of a magnet or magnetic yoke for a
Magnetic Resonance Imaging apparatus according to this
invention.
[0060] FIG. 2 is a plan view of a portion of a magnet or magnetic
yoke for a Magnetic Resonance Imaging apparatus according to this
invention.
[0061] FIG. 3 is a side view of a detail of a magnet or magnetic
yoke for a Magnetic Resonance Imaging apparatus according to this
invention.
[0062] FIG. 4 is a plan view of a magnetic pole piece for a
Magnetic Resonance Imaging apparatus according to this
invention.
[0063] FIG. 5 is a side view of a portion of a magnetic pole piece
for a Magnetic Resonance Imaging apparatus according to this
invention.
[0064] FIG. 6 is a further side view of a magnetic pole piece for a
Magnetic Resonance Imaging apparatus according to this
invention.
[0065] FIGS. 7, 8 are plan views of two types of sheets for a
magnetic pole piece for a Magnetic Resonance Imaging apparatus
according to this invention.
[0066] FIG. 9 is a plan view of a sheet pack for a magnetic pole
piece for a Magnetic Resonance Imaging apparatus according to this
invention.
[0067] FIG. 10 is a plan view of a plate of a magnetic pole piece
for a Magnetic Resonance Imaging apparatus according to this
invention.
[0068] FIG. 11 is a side view of a Magnetic Resonance Imaging
apparatus according to this invention.
[0069] FIG. 12 is a side view of a construction detail of a
Magnetic Resonance Imaging apparatus according to this
invention.
[0070] FIG. 13 is a rear view of a Magnetic Resonance Imaging
apparatus according to this invention, with the two pole pieces on
the same horizontal plane.
[0071] FIG. 14 is a rear view of a Magnetic Resonance Imaging
apparatus according to this invention, with the two pole pieces on
the same vertical plane.
[0072] FIG. 15 is a perspective view of a shaft for a Magnetic
Resonance Imaging apparatus according to this invention.
[0073] FIG. 16 is a perspective view of a pole piece for a Magnetic
Resonance Imaging apparatus according to this invention, where the
peripheral heating bands and the two flat D-shaped heaters are
visible.
[0074] Particularly, FIG. 1 shows a magnet for a Magnetic Resonance
Imaging apparatus which has three open sides and delimits a patient
receiving area. The magnet is composed of three yoke elements, i.e.
two parallel magnetically permeable yoke elements 1, 2 and a
transverse yoke element 3 for connection of these two parallel
elements 1 and 2, whereby the magnet has a C shape.
[0075] In an apparatus with an overhangingly supported magnet,
which essentially has a C- or laterally inverted U-shape, problems
arise when the magnet is required to be rotated.
[0076] In a Magnetic Resonance Imaging apparatus whose magnet keeps
the same orientation relative to the gravitational field, the
displacement of pole pieces is typically not a problem, as the
forces acting in the pole piece support members are compensated for
during fabrication.
[0077] However, when a magnet rotation is required, e.g. for the
examination of a body in certain positions or of certain body
parts, problems often arise, as the magnet of the apparatus is
essentially overhangingly supported by the support structure and is
quite heavy. The rotation causes great bending stresses to act on
the two arms of the C-shaped magnet, due to the heavy weights being
involved. During and after the rotation of the magnet, a situation
is generated in which the bending stress causes such deformations
that the pole pieces of the magnet are substantially displaced
relative to each other, whereby the homogeneity of the magnetic
field is altered, and the quality of body images is affected.
[0078] More generally, the pole piece support structure may be said
to be mainly acted upon by two forces: the gravity force and the
force of the magnetic field generated between the pole pieces.
[0079] The force of the magnetic field between the pole pieces
tends to bend the two arms of the C- or U-shape toward each other
and is not affected by the spatial arrangement of the support
structure, but remains essentially constant regardless of the
spatial arrangement and the rotation of the apparatus.
[0080] However, the gravitational force is directed downwards, as
is known, and causes different stresses on the structure depending
on the position of the support structure, i.e. generally the
magnetic yoke. In fact, when the two arms of the C shape are one
above the other, with vertically superimposed pole pieces, the
force of the gravitational field acts in such a manner that the
upper arm tends to bend downwards, whereas the lower arm tends to
move toward the floor. In case of a 90.degree. rotation, when the
arms of the C shape are in side-by-side positions, gravity-induced
stresses cause both arms to deform toward the floor in the same
direction. All intermediate positions of the magnet, between the
two above mentioned extreme conditions, cause a dramatic change of
the stresses acting on the arms of the C-shape.
[0081] These two forces, i.e. the force of the gravitational field
and the force of the magnetic field sum up during normal operation
and generate a variable stress depending on which position is taken
by the magnet: if the arms of the C shape are one above the other,
the force of the magnetic field tends to partly compensate for the
gravity force, thereby reducing the stress on the lower arm of the
C shape. After a 90.degree. rotation, the force of the magnet field
has no effect on the gravity force, as it acts perpendicular
thereto, whereby two deformations along perpendicular planes are
produced: the gravity force causes a downward deformation on the
vertical plane, whereas the magnetic force acts on the horizontal
plane, and draws the two arms of the C shape toward each other.
[0082] For a magnet that is not intended to be rotated, a
compensation for the above deformations may be attempted during
design and fabrication, e.g. by a construction which preventively
accounts for the expected magnet deformation, or by the well-known
shimming mechanism, which corrects the magnetic field and makes it
uniform and parallel even in case of non homogeneous and parallel
lines of flux, due to the gravitational force and the magnetic
field force.
[0083] Furthermore, for weight withstanding purposes, prior art
overhangingly mounted apparatuses which are not designed for
rotation are often loaded in a mechanical pre-stress condition, a
bending stress essentially contrary to the stress that the
apparatus is expected to receive in the assembled condition being
induced by suitable design and assembly arrangements, to limit the
deformation of the apparatus after installation.
[0084] The arrangements used in prior art for non 20, rotating
magnets are not applicable to rotating magnets, as even the
slightest rotation of a C-shaped magnet causes an overall change of
the mechanical stress conditions, as mentioned above.
[0085] Therefore, a prior art apparatus with a rotating C-shaped
magnet often has the drawback of providing either a small imaging
volume or a low image quality.
[0086] As shown in FIG. 1, in both parallel elements 1, 2, the end
portion associated to the transverse connection element 3 has a
transverse wall for connecting the two parallel elements with the
transverse element 3, which has an end step 101, 102, for
engagement of a corresponding end 103, 203, of the transverse
element 3.
[0087] The elements 1 and 2, which are perpendicular to and
overhangingly supported by the transverse element 3 are secured
thereto by suitable fastener means 401, 402, which are oriented
perpendicular to the longitudinal extension of the transverse wall
3 and engage the two parallel elements 1, 2 with the transverse
element 3 in the raised thickness of this end step 101, 102.
Referring now to FIGS. 1, 2, 3, further fastener means 503 are
shown which are oriented parallel to the longitudinal extension of
the transverse wall 3, and engage the two parallel elements 1, 2
with the transverse element 3 at the end step 101, 102.
[0088] The fixation between the elements of the C-shaped structure
of the magnetic yoke is further reinforced by reinforcement member
5, which are positioned in the inwardly facing corners between the
parallel elements 1, 2 and the transverse element 3, and which are
retained in position between the parallel elements 1, 2 and the
transverse element 3 by fastener means 105, 205.
[0089] Particularly, in the preferred embodiment of the Figures,
the reinforcement members 5 are each formed by a triangular member
and may be provided in a variable number depending on the
sturdiness required from the magnet. The weight and rigidity of the
structure may be advantageously varied by providing a single
triangular member 5 extending essentially along the whole parallel
element and transverse element coupling length, or one or more
preferably triangular reinforcement ribs 5, 6, which have a
definitely lighter weight.
[0090] Particularly, in a preferred embodiment, two triangular
reinforcement ribs are provided for each parallel
element/transverse element pair, which is a good compromise between
rigidity and lightness of the assembly.
[0091] The fastener means are preferably provided in the form of
screw fasteners, preferably having a head with a greater diameter
than the cylindrical shank of the screw. Thus, the various parts of
the magnet may be conveniently centered and assembled in the proper
position.
[0092] If the magnet, and therefore the pole pieces of a Magnetic
Resonance Imaging apparatus are intended to be rotated, the pole
pieces shall be constantly in a proper relative position, to form a
highly homogeneous magnetic field and provide high quality images.
For a rotating magnet, there is the risk that, as the direction of
the gravity force on the magnetic yoke changes, due to the heavy
weight of the latter and to the overhanging support of its parallel
elements, small displacements may occur between the parallel
elements and between the pole pieces, thereby causing unevenness
and dishomogeneity of the magnetic field.
[0093] In all Magnetic Resonance Imaging apparatuses in which the
magnet structure always has the same orientation with respect to
gravity, any offset from the ideal position of the pole pieces,
caused by gravity or by mutual magnetic attraction between the
opposed pole pieces is accounted and compensated for during
fabrication.
[0094] However, a magnet according to the present invention assures
that no or almost no relative displacement occurs between parallel
elements, and between the pole pieces, whereby the magnetic field
remains uniform and homogeneous even when the magnet is rotated,
and assures that the image of the body under examination has a good
quality.
[0095] The magnet of this invention advantageously and essentially
allows to avoid any relative displacement between the parts which
compose the magnet so that, even upon rotation of the magnet, the
magnetic field is essentially unaltered and optimal for a proper
examination of the body in any position thereof, at least in the
imaging volume. The structure is highly stable, and the deformation
of the parts which compose the C-shaped magnet is
insignificant.
[0096] According to a further characteristic of this invention, as
shown in FIGS. 11 and 12, the magnet is rotatably handled about an
axis perpendicular to at least one axis of the cavity opening,
particularly in the preferred arrangement of the figures the axis
of rotation essentially passes through the center of the imaging
volume of the apparatus, where imaging volume, as used herein,
refers to the portion of the magnet-defined cavity in which the
magnetic field has the best characteristics for imaging the body
under examination.
[0097] In order that the magnet may be rotated about the preferred
axis, it has a coupling flange 30 in its transverse wall opposite
the cavity, for coupling the shaft 31 to the transverse wall of the
magnet, said transverse wall being rotatably mounted to a support
case 32 on rotatably and translatably supporting means 32, such as
slide/rotary guides and/or roller bearings.
[0098] The coupling flange 30 and the support case part 32 have
mutually cooperating lock/release means for locking/releasing the
rotation, which allow to change from a magnet rotation condition to
a magnet stop condition.
[0099] The means for locking/releasing the magnet are provided in
the form of two opposed and interconnected ring gears 36, 37,
whereof one is integral with the support case 32 and does not
rotate, and the other is integral with the shaft 31 and rotates
therewith. The ring gears are maintained in a stable mutual
engagement condition by the force exerted by an elastic member, and
means are provided for moving the two ring gears to a disengagement
condition against the action of the elastic member 39.
[0100] Particularly, according to a preferred embodiment, the
elastic member is provided in the form of a Belleville spring,
which ensures a highly strong attachment, adapted to maintain the
two ring gears in a mutual engagement condition.
[0101] The ring gears 36, 37 preferably have front teeth, which
provide a more compact structure and a better and more accurate
mesh between the teeth thereof.
[0102] As shown in FIG. 12, the ring gear 36 associated to the
shaft 31 is integral with the coupling flange 30 and its teeth are
directed opposite the magnet.
[0103] While the magnet is moving, its rotary motion is transmitted
to the shaft 31 by a shaft handling grip, which is formed by a
radial lever 63 which is rotatably and/or translatably integral
with the shaft; this lever is articulated to the rod of a hydraulic
or pneumatic cylinder actuator 64, i.e. a linear actuator, which
generates a rectilinear motion, to be turned into a rotary motion
of the shaft by the action of the radial lever.
[0104] The shaft 31 is preferably associated to the support case 32
by means of roller bearings, which are preferably placed at the
ends of the shaft.
[0105] The linear actuator is itself connected by its lower portion
to the case and, depending on the desired embodiment, may be
stationary or translatable on a suitable guide with respect to
it.
[0106] In one embodiment, the radial lever 63 is coupled with the
rod of the actuator 64 by sliding means, which slide in the axial
shaft translation direction, e.g. the lever has a perforated and
thinned terminal, as shown in FIG. 15 which is mounted on a bar
that is held at its ends by a terminal fork 90 for connection with
the rod. The fork 90 has such a width that the bar held thereby is
at least as long as or slightly longer than the translation stroke
of the shaft. Therefore, a translational motion of the shaft and
the associated radial lever does not involve an identical
translational motion of the actuator, which stands still with
respect to the case, as the radial lever 63 slides on the rod which
is retained by the fork 90 of the linear actuator 64.
[0107] In an alternative embodiment, the linear actuator has a
slide guide, not shown, at its end associated to the support case,
for allowing translational motion of the actuator, and the rod of
the actuator has one degree of freedom, particularly of rotation,
with respect to the radial lever. Hence, as the shaft is
translated, it also drives into translation the linear actuator,
which follows the shaft by translating on the slide guide that
connects it to the support case.
[0108] In a further alternative embodiment, the linear actuator is
connected to the support case or to the radial lever with the
interposition of a joint which allows it to tilt to accommodate the
shaft translation. This is possible because such translational
motion extends for so little, i.e. in a range of a few millimeters,
that the substantial verticality of the linear actuator is not
dramatically affected. As an alternative, the radial lever may be
designed to have a joint which allows it to tilt relative to the
shaft, to accommodate the translational motion of the shaft with
respect to the actuator.
[0109] As shown in FIGS. 11 and 12, the ring gears 36, 37 and the
Belleville spring 39 may be respectively displaced by hydraulic or
pneumatic displacement means, and may be stressed thereby in the
axial direction of the shaft 31.
[0110] Moreover, as shown in FIGS. 11 and 12, the end of the shaft
31 opposite the magnet is provided in the form of a hydraulic or
pneumatic lock/release cylinder actuator, whose rod 50 projects out
of the end of the shaft 31, and is mounted in such a manner as to
be unable to rotate relative to the shaft 31 and is alternately
subjected to the action of elastic means, such as the Belleville
spring 39, which oppose the shaft translation toward disengagement
of the ring gear teeth. The end of the shaft 31 opposite the magnet
has a coaxial dead recess, which acts as a chamber 51 for a
hydraulic or pneumatic lock/release cylinder actuator, and receives
a piston 52 therein, which is mounted at the inner end of the rod
50, whereas the cylindrical recess is tightly closed by an end
portion 54 at the head of the shaft, a spring 39 being provided
between the piston 52 and the end portion 54, whereas the piston 52
tightly projects out of this end portion.
[0111] Therefore, when the magnet has to be rotated, the chamber of
the lock/release actuator cylinder is, for instance, filled with
oil whereby the Belleville spring is stressed in the compression
direction by the hydraulic lock/release actuators. This causes the
shaft to be translated relative to the support case, and
particularly, in the embodiment of FIG. 12, the shaft moves toward
the magnet, thereby bringing the ring gear 37 integral with the
magnet and with the shaft, to disengagement from the ring gear 36
integral with the support case 32. This releases the rotation of
the magnet connected to the shaft, which is driven by the radial
lever moved by the linear actuator. Once the rotation of the magnet
is completed, the Belleville spring is released and brings the ring
gears 36 and 37 into a mutual engagement position, thereby actually
locking the rotation.
[0112] The relative position of the rotating shaft and the case may
be controlled by sensors and/or photocells which ensure that shaft
rotation only stops when the facing teeth of the ring gears 36 and
37 are in a proper mutually meshed position, so that meshing occurs
with no shocks or undesired small rotations. As an alternative
thereto or in combination therewith the magnet extreme positions
may be also monitored, so that shaft rotation automatically stops
when the shaft reaches either extreme position, i.e. when the
magnet is in either C- or U-shaped position.
[0113] This preferred arrangement provides a Magnetic Resonance
Imaging apparatus which has a simple construction and is highly
fail-safe, as the magnet may be only released when it is stressed
by the lock/release piston which moves the two ring gears into
disengagement and allows free rotation of the magnet. Hence, in
case of a sudden failure of the apparatus or a sudden power
failure, the action of the Belleville spring automatically moves
the ring gears into a teeth mesh position, thereby causing the
magnet to be immediately locked in position, and preventing it from
uncontrollably and undesirably rotating.
[0114] A further advantage as compared with prior art apparatuses
is that any magnet lock/release and rotation action is performed by
linear actuators, which have a simpler construction and ensure a
higher reliability at lower costs as compared with normal prior art
actuators.
[0115] In another preferred embodiment, each pole piece as shown in
FIGS. 4 to 6 comprises at least one magnetized layer 6, at least
one magnetically permeable layer or plate 9, wherein the
magnetically permeable plate 9 has a substantially circular shape
and said magnetized layer 6 is formed by a plurality of adjacent
and/or superimposed magnetized blocks 106, which form a polygon
essentially approximating the circular shape of the magnetically
permeable plate 9.
[0116] FIG. 4 shows fastener posts 7 for securing the pole piece 8
to the inner face of the corresponding parallel yoke element 1, 2,
which are situated in the margin between the polygon and the
circumference approximated by the polygon. The fastener posts 7 are
placed in the recesses of the stepped peripheral edges formed by
the blocks 106, which have a polygonal, particularly cubical shape
or a rectangular section, and together form a polygonal layer with
stepped peripheral edges.
[0117] The posts 7 preferably have a tubular shape and are placed
in centering recesses 206 for positioning the posts 7, wherein
corresponding post ends may be introduced, the centering recesses
206 being complementary in size to the outside diameter of the
posts 7 and/or to the outside diameter of the associated end of the
post 7, to allow proper insertion thereof, and to add rigidity to
the assembly. The pole piece support structure also has similar
centering recesses 501 on the surface facing toward the pole piece,
for the fastener posts 7 of the pole piece. The fastener posts 7
have a tubular shape and contain a hole for the passage of an
associated fastener means, such as a bolt and/or a screw.
Furthermore the fastener posts 7 of the pole piece have a reduced
or increased diameter at their free ends, corresponding to the
diameter of the associated centering recesses 206 and/or to the
corresponding diameter of the positioning recesses, to act as
spacers with a predetermined stable size. The posts are pressed
against the corresponding surfaces of the bottom sides of said
recesses and actually remain compressed against them.
[0118] After a rotation that brings the pole pieces onto the same
horizontal plane, the gravity force acts tangentially to the pole
piece surface and cause a stress perpendicular to the stress
obtained when the pole pieces are disposed on the same vertical
plane. The posts that act as spacers and are connected by their end
portions in the centering recesses prevent the pole pieces, and the
components thereof, from moving relative to each other, as the
gravity force changes.
[0119] As shown in FIGS. 6 to 9, a sheet pack 11 is provided in the
pole piece, which is formed by at least one metal sheet 13 with
cuts thereon, which is positioned on the surface of the
magnetically permeable plate 9 opposite the surface of the plate
which is turned toward the magnetized layer 6 and is used to obtain
a more homogeneous magnetic field.
[0120] A sheet pack 11 formed by multiple superimposed sheets is
preferred. Particularly, as shown, the magnetically permeable plate
9 has a cup shape, having at least one, preferably two peripheral
steps, which form a raised edge projecting out of the plate surface
turned opposite the magnetized layer 6, so that the sheet pack lies
on the bottom 12 of the cup-shaped plate and under the known
shimming plate 70 which peripherally lies on the second step, hence
at a distance from the bottom of the plate, equaling the thickness
of the sheet pack 11. In a preferred embodiment, the pole piece has
an electromagnetic shield 13 which is placed in the end portion of
the pole piece opposite the magnetized blocks, and over the
shimming plate, so that the two electromagnetic shields of the two
opposed, facing pole pieces are essentially turned toward the
patient table 99.
[0121] As shown in FIGS. 7, 8, 9, the sheet pack 11 has a plurality
of cuts, the cuts of one sheet being located in different positions
with respect to the cuts of the preceding or subsequent sheet in
the sheet pack 11. This is achieved by providing that the cuts on
each of the metal sheets are essentially non coincident between
pair sheets 15 and odd sheets 16, with reference to the sheet
positioning order in the sheet pack 11, and preferably the cuts are
straight. This arrangement adds homogeneity to the magnetic
field.
[0122] Moreover, in order to achieve a good image quality, the
temperature of the magnetic pole pieces should be maintained
constant, as any temperature change might cause an image quality
drop.
[0123] Thus, a further object of this invention is to provide pole
pieces in which pole piece heating and temperature monitoring are
optimized, whereby a preferred embodiment provides pole piece
heater devices and pole piece temperature sensors; also, the pole
piece may be adapted to receive the cables for the pole piece
heater devices and temperature sensors. Suitable apertures are
formed in the peripheral edges of the pole piece for the passage of
the cables for the heaters and/or the temperature sensors, so that
the pole piece is as compact as possible.
[0124] According to a preferred embodiment, as shown in FIG. 10,
the apertures are two passages 18, 19 formed on diametrically
opposite locations of the plate 9 to prevent the passage of cables
through the pole piece.
[0125] Particularly, electrical heaters are provided, which are in
contact with said plate 9 and/or with the magnetized material
and/or with the sheet pack 11, as shown in the figures.
[0126] Furthermore, a particularly capillary and uniform heating
arrangement may be provided by positioning at least some of these
heaters within the magnetized layer 6 and particularly between
adjacent magnetized blocks 106.
[0127] In a preferred embodiment, which is shown in FIG. 16, the
heaters may be placed on the externally accessible pole piece
surface, which allows easier service and maintenance thereof.
Particularly these heaters may be peripheral heaters 90, placed on
the peripheral side surface of the edge of the pole piece and are
not separate heaters but are provided continuously over the whole
peripheral side surface of the pole piece, to form a peripheral
heating edge.
[0128] Moreover, a more homogeneous heating arrangement may be
obtained by providing flat heaters 91 over at least one face of the
pole piece, these heaters preferably having the shape of a sector
of a circle, more preferably a D shape, and being spaced over the
face of the pole piece, to form an area therebetween which is not
directly heated, as shown in FIG. 16.
[0129] In order that temperature detection and monitoring may be as
accurate as possible, the temperature detecting and monitoring
sensors 97, as shown in FIG. 16, are positioned within the pole
piece, and particularly temperature sensors may be provided level
with the plate 9 and/or the magnetized material and/or the sheet
pack 11. Furthermore, some of the temperature sensor means may be
arranged to be positioned within the magnetized layer 6, preferably
between adjacent magnetized blocks 106.
[0130] Preferably, there may be two of these temperature sensors
97, 97', placed inside the pole piece, particularly between the
blocks and the sheet pack of the pole piece, in the area that is
not directly heated by the two D-shaped heaters, as shown in FIG.
16. Therefore, temperature is sensed in an area that is not
directly heated by heaters, whereby a more reliable detection is
obtained, the temperature of the whole pole piece being more
indicative than the temperature of a portion of it.
[0131] Also, the provision of two sensors 97 advantageously avoids
the need of dismantling and reassembling the pole piece and of
subsequently recalibrating the shimming plate, in case of failure
of one of the two pole pieces, because, with a very little starting
cost, the apparatus may be arranged to continue operating with a
single sensor.
[0132] Furthermore, the provision of two sensors allows temperature
monitoring and detection in particularly difficult areas of the
pole pieces, such as the areas of the magnetized blocks, with the
highest accuracy, thereby assuring a good image quality.
[0133] FIG. 11 further shows the lateral slide guides 95 for the
patient table 99 which, in a preferred embodiment, are provided in
the form of a vertical support tab 94 which extends toward the pole
piece from the sides of the patient table, said support shoulder or
tab being associated to a guide and slide combination, which is
preferably formed by a longitudinal slide having a cylindrical head
93, cooperating with a concave guide 92 which has a corresponding
open profile. Thus the total height of the patient table 99 and the
pole piece is considerably smaller than the heights of prior art
patient tables and pole pieces, as guides are positioned sideways
and the volume does not extend vertically.
[0134] The longitudinal slide 93 is overhangingly supported toward
the median area of the patient table surface and below said patient
table surface by lower side tabs, branching off the two opposite
longitudinal sides of the patient table. The guide 92 is carried by
the shoulder which is oriented parallel to the slide and/or to the
side tabs of the patient table, and this guide is open on the side
turned toward the inner face of the tabs which carry the
longitudinal slide 93. A guide 92 and slide 93 combination is
provided on each longitudinal side of the patient table.
[0135] The guide/slide combination may be advantageously provided
in the form of a continuous guide which is coupled to a
discontinuous slide or vice versa, to further and advantageously
reduce the weight of the patient support device, which is intended
to rotate with the magnet and therefore must have as light a weight
as possible.
* * * * *